Explain post-weld heat treatment (PWHT) and when it is used.

• A. PWHT is controlled heating and cooling after welding to relieve residual stresses and improve ductility and toughness; used on certain steels and critical welds.
• B. PWHT is the process of extending the welding duration to reduce cost.
• C. PWHT is a method for applying protective paint after welding.
• D. PWHT is the procedure of cutting the weld with a saw after welding.

Answer: (A)

Post-weld heat treatment is a controlled process of heating the welded joint after welding to a defined temperature, holding it there, and then cooling in a controlled way. The goal is to relieve the residual stresses generated by the rapid heating and cooling during welding, which can otherwise cause distortion or cracking. It also alters the microstructure in the heat-affected zone to reduce hardness and increase ductility and toughness, improving the weld’s performance in service. PWHT is typically used on certain steels and critical welds—especially thick sections or materials prone to embrittlement or hydrogen cracking—where codes and standards (like those for pressure vessels, pipelines, or structural welds) require it to ensure safety and reliability.